Matrix metalloproteinases (MMPs) are a group of matrix degrading enzymes whose aberrant or excessive expression can lead to a variety of tissue destructive diseases. Less is known about the normal physiologic functions of MMPs. We present data that macrophage elastase (MMP-12) is the only MMP that has direct antimicrobial activity. MMP-12 acts within the lung macrophage as the first line of defense against microbes within the alveolar space. MMPs are well known for their roles in promoting tumor progression. However, with the discovery of angiostatin, an antiangiogenic proteolytic fragment of plasminogen, it became clear that proteinases can be involved in limiting tumor growth. We present evidence that MMP-12 plays a major role in limiting tumor growth within the lung. This property might have clinical importance since at least 6 phase 3 trials using MMP inhibitors for cancer therapy and two for arthritis were stopped last year related to this under-appreciated property of certain MMPs to limit tumor growth. To further define the role of macrophages and MMP-12 in host defense against bacteria and tumors in the lung, we propose to: 1. Test the hypothesis that MMP-12 represents a novel macrophage-mediated intracellular antimicrobial agent. We provide preliminary data that MMP-12-/- mice have a poorer outcome in response to S. aureus pneumonia, MMP-12-/- macrophages have impaired intracellular killing of S. aureus, and show that MMP-12 has direct antimicrobial capacity. This activity is independent of catalytic capacity and involves the non-catalytic C-terminal domain. Studies are proposed to define the spectrum of bacteria influenced by MMP-12. We will also define the structural components of MMP-12 responsible for this activity. 2. We will extend the hypothesis that MMP-12 interferes with tumor growth via inhibition of angiogenesis and further define potential mechanisms of action. We provide preliminary data that MMP-12 is required to maintain dormancy of Lewis lung cell carcinoma (LLC) metastases. This activity appears related to inhibition of angiogenesis. This is not merely due to generation of angiostatin. Additional antiangiogenic protein fragments play a role, and we postulate that MMP-12 also interferes with MMP-2-mediated promotion of tumor growth. MMP-12 might do this by cleavage of MMP-2 as well as by competition with MMP-2 for endothelial cell and tumor cell binding through its C-terminal domain. 3. We will determine the role of macrophages in lung development, bacterial infection, and tumor progression. We will take advantage of MMP-12 macrophage specific expression and complete generation of diphtheria toxin (DT) "knock-in" to the MMP-12 locus. We hypothesize that this will result in mice deficient in lung (and peritoneal) macrophages, and that these mice will undergo normal lung development. If this hypothesis is correct, then the mice will be used to study the requirement of macrophages in host defense and inflammation. If the mutation is lethal or not fully deficient in pulmonary macrophages, then lung-specific transgenic mice will be used to inducibly express DT in lungs of mature mice.